*4.2. Mechanical and Tribological Properties*

Reactive deposition in both Ar-N2 and Ar-C2H4 resulted in the enhancement of mechanical properties and tribological performance of the coatings. The increase of mechanical properties can be partially attributed to the growth of internal stresses (Table 2). In the case of N-containing coatings, this effect might be associated with the formation of a crystalline/amorphous phase based on TaN [38,39], as well as a decrease in the size of crystallites [40]. In C-doped coatings, the rise in hardness is due to the formation of a hard DLC-phase, as well as the positive effect of microstructural modification via the introduction of carbon into the composition of the coating. The complete understanding of structural features and phase composition of reactive coatings requires additional studies, which we plan to do next.

All the coatings investigated in this work were characterized by a high friction coefficient when relatively soft material (steel 100Cr6, H = 8 GPa) was used as a counterpart. The surface of the coating is much harder (H = 12–30 GPa), and has high adhesion to steel, similar to [5]. The distinct behavior of coating 5 at the initial part of testing distance can be associated with the positive role of free carbon, which could be released owing to the oversaturation of the crystalline carbide phase and plays the role of a solid lubricant during friction [45,46]. Thus, coating 5 obtained at a flow rate of 10 sccm C2H4 had the lowest friction coefficient *f* in contact with the 100Cr6 steel ball. Under more stringent conditions (Al2O3 counterpart), the lowest *f* was demonstrated by sample 4 deposited at 5 sccm C2H4. The coating obtained at 5 sccm N2 had a stable friction coefficient against both counterparts (100Cr6 steel and Al2O3). The transition to reactive sputtering leads to a decrease in the coatings' wear rate, which coincides with the results of other works [41,47]. The best wear resistance was recorded for coatings 2 and 4, obtained at 5 sccm in N2 and C2H4. This effect can be attributed to the presence of amorphous solid lubricant phases.
